In the control of machine tools, it is important for operator safety to activate the machine tool only when a safety gate is in place to protect the operator from physical harm. The operator may be injured by the machine tool moving and inadvertently striking the operator. A limit switch is often used to prevent the machine tool from moving before the safety gate is in place. A number of limit switches have been devised in the past for the control of a machine tool. Many attempts in the past have been made to adapt a mechanically operated toggle switch to sense the presence of a closed safety gate. A disadvantage of a mechanically operated toggle switch is that is can be defeated by operator action. For example, a toggle switch may be taped closed, or may be wired closed. The machine tool can then be operated with the safety gate in an unsafe position.
A problem addressed by the present invention is to make a safety gate limit switch which is difficult to defeat by action of the operator. Akehurst, U.S. Pat. No. 4,349,814, issued on Sept. 14, 1982, shows the use of Hall effect magnetic field sensors to make a safety gate limit switch. Akehurst uses Hall effect transducers arranged to require magnetic poles of opposite polarity to activate his limit switch. A disadvantage with this arrangement is that a short bar magnet having a north pole and a south pole at opposite ends could be placed in proximity to the magnetic field detector and thereby expose the two Hall effect transducers to the proper magnetic fields for activating the switch. A further problem addressed by the present invention is to provide a magnetic field sensing arrangement which provides greater safety against being overridden by any simple action of the operator, and also to provide a detector system and logic circuit for the invention.